wiki:

EncodingForStreamingSites

Encoding for streaming sites

Such as twitch.tv, ustream.tv, YouTube Live, and other RTMP(E) streaming providers. Since FFmpeg development is very active it is recommend that you follow a compilation guide to take advantage of bug fixes and new features.

Note: Do not blindly run any of these commands. You must customize your -crf, -preset, -maxrate, -bufsize, and -g as shown in this guide (do not worry – it is not difficult).

-crf

Consider this to be as close as you can get to a "constant quality" mode. Range is 0-51: 0 is lossless, 18 is often considered to be roughly "visually lossless", 23 is the default, and 51 is the worst. Generally you want to use the highest value that still provides an acceptable quality. If you do not know what to choose then 23-28 will probably be acceptable for streaming.

-preset

This provides the compression to encoding speed ratio. Use the slowest preset you can: ultrafast,superfast, veryfast, faster, fast, medium, slow, slower, veryslow, placebo. Try -preset veryfast if you are unsure of what to choose, then watch the console output or the video to see if the encoder is keeping up with your desired output frame rate: if it is not then use a faster preset and/or reduce your -video_size.

-maxrate

Anytime you are encoding video with bandwidth as a limiting factor you should be using VBV (Video Buffer Verifier) with the -maxrate and -bufsize options:

  • Assuming your upload rate is 1024kbit/s (1 megabit/s), and assuming you can reliably utilize 80% of that = 820 kbit/s. Audio will consume 128 kbit/s (64k/channel for stereo, but you can of course use a different audio bitrate) leaving ~692 kbit/s for video: this is your -maxrate value.
  • If you have a sane upload rate, or do not know what to choose then a -maxrate value of up to 3000k-4000k will probably be fine if your upload rate can handle it (depending on your input complexity and -video_size). Refer to your streaming service for any limitations that may apply.
  • Note that the claimed data rate pay your ISP for may not actually be what you get. You can use a site like speedtest.net to test.

-bufsize

-bufsize sets the buffer size, and can be 1-2 seconds for most gaming screencasts, and up to 5 seconds for more static content. If you use -maxrate 960k then use a -bufsize of 960k-1920k. You will have to experiment to see what looks best for your content. Refer to your streaming service for the recommended buffer size (it may be shown in seconds or bits).

-g

Use a 2 second GOP (Group of Pictures), so simply multiply your output frame rate * 2. For example, if your input is -framerate 30, then use -g 60.

MP3 or AAC

If you compiled ffmpeg with support for libfdk_aac, then you can use that for your audio. If not, then use -acodec libmp3lame -ar 44100 as shown in many of these examples. MP3 in FLV is limited to specific audio rates, but you can omit -ar if you are encoding to AAC. See the FFmpeg and AAC Encoding Guide for more information.


Streaming your desktop

Without scaling the output

If you want the output video frame size to be the same as the input:

$ ffmpeg -f alsa -ac 2 -i hw:0,0 -f x11grab -framerate 30 -video_size 1280x720 \
-i :0.0+0,0 -vcodec libx264 -preset veryfast -maxrate 1984k -bufsize 3968k \
-vf "format=yuv420p" -g 60 -c:a aac -b:a 128k -ar 44100 \
-f flv rtmp://live.twitch.tv/app/<stream key>

Scaling the output

If you want the output video frame size to be smaller than the input then you can use the scale video filter:

$ ffmpeg -f alsa -ac 2 -i hw:0,0 -f x11grab -framerate 30 -video_size 1680x1050 \
-i :0.0+0,0 -vcodec libx264 -preset veryfast -maxrate 3000k -bufsize 3000k \
-vf "scale=1280:-1,format=yuv420p" -g 60 -c:a aac -b:a 128k -ar 44100 \
-f flv rtmp://live.twitch.tv/app/<stream key>

The -1 in the scale filter example will automatically calculate the correct value to preserve the height. In this case the output will have a frame size of 1280x800.

With webcam overlay

This will place your webcam overlay in the top right:

$ ffmpeg -f x11grab -video_size 1680x1050 -framerate 30 -i :0.0 \
-f v4l2 -video_size 320x240 -framerate 30 -i /dev/video0 \
-f alsa -ac 2 -i hw:0,0 -filter_complex \
"[0:v]scale=1024:-1,setpts=PTS-STARTPTS[bg]; \
 [1:v]scale=120:-1,setpts=PTS-STARTPTS[fg]; \
 [bg][fg]overlay=W-w-10:10,format=yuv420p[out]"
-map "[out]" -map 2:a -vcodec libx264 -preset veryfast \
-maxrate 3000k -bufsize 4000k -c:a aac -b:a 160k -ar 44100 \
-f flv rtmp://live.twitch.tv/app/<stream key>
  • You can see additional details your webcam with something like: ffmpeg -f v4l2 -list_formats all -i /dev/video0 or with v4l2-ctl --list-formats-ext. See the documentation on the video4linux2 (v4l2) input device for more info.
  • Your webcam may natively support whatever frame size you want to overlay onto the main video, so scaling the webcam video as shown in this example can be omitted (just set the appropriate v4l2 -video_size and remove the scale=120:-1,).

This will place your webcam overlay in the top right, and a logo in the bottom left:

$ ffmpeg -f x11grab -video_size 1680x1050 -framerate 30 -i :0.0 \
-f v4l2 -video_size 320x240 -framerate 30 -i /dev/video0 \
-f alsa -ac 2 -i hw:0,0 -i logo.png -filter_complex \
"[0:v]scale=1024:-1,setpts=PTS-STARTPTS[bg]; \
 [1:v]scale=120:-1,setpts=PTS-STARTPTS[fg]; \
 [bg][fg]overlay=W-w-10:10[bg2]; \
 [bg2][3:v]overlay=W-w-10:H-h-10,format=yuv420p[out]"
-map "[out]" -map 2:a -vcodec libx264 -preset veryfast \
-maxrate 3000k -bufsize 4000k -c:a aac -b:a 160k -ar 44100 -b:a 128k \
-f flv rtmp://live.twitch.tv/app/<stream key>

Streaming a file

$ ffmpeg -re -i input.mkv -vcodec libx264 -preset veryfast -maxrate 3000k \
-bufsize 6000k -pix_fmt yuv420p -g 50 -c:a aac -b:a 160k -ac 2 \
-ar 44100 -f flv rtmp://live.twitch.tv/app/<stream key>

Encoding a file for streaming

If your computer is too slow to encode the file on-the-fly like the example above then you can re-encode it first:

$ ffmpeg -i input.mkv -vcodec libx264 -preset medium -maxrate 3000k -bufsize 6000k \
-vf "scale=1280:-1,format=yuv420p" -g 50 -acodec libmp3lame -b:a 128k -ac 2 -ar 44100 file.flv

Then stream copy it to the streaming service:

$ ffmpeg -re -i file.flv -codec copy -f flv rtmp://live.twitch.tv/app/<stream key>

Notes

  • You can use xwininfo | grep geometry to select the target window and get placement coordinates. For example, an output of -geometry 800x600+284+175 would result in using -video_size 800x600 -i :0.0+284,175. You can also use it to automatically enter the input screen size: -video_size $(xwininfo -root | awk '/-geo/{print $2}').

Getting help

Always use a recent ffmpeg. See the compilation guides for more information.

If you need additional help then send a message to the ffmpeg-user mailing list (see MailingListEtiquette first) or hang out in the #ffmpeg IRC channel on Freenode.

Last modified 3 months ago Last modified on Mar 10, 2016, 9:07:50 PM